There exists an increasing demand for reduction in the amount of pulp used in paper making for protection of the global environment and consequently in the weight of paper and increased used of waste paper pulp. However, the reduction in the amount of pulp used in paper leads to a paper thinner and reduced in opacity, resulting in deterioration in the quality of the paper. In addition, weight saving by reduction in the amount of pulp used for paper making decreases the stiffness of the resulting paper, which is unfavorable especially for papers demanding a higher stiffness such as cardboard and the like, which is proportional to the thickness to the third power. On the other hand, increased use of waste paper pulp leads to deterioration in brightness due to the ink remaining in the waste paper pulp or the like and in opacity due to the decrease in paper thickness caused by the wear in pulp bulky value during recycling. As a result, reduction in the amount of pulp and increase in the amount of waste paper pulp used in paper in combination leads to further decrease in the opacity and brightness of the paper obtained. Further, deinking and bleaching of waste paper pulp, which is the primary cause of the deterioration in brightness, for improvement in the brightness unfavorably leads to further deterioration in the opacity of paper.
Various bulky value-improving processes have been proposed to reduce the weight of paper, but prevent the thickness from decreasing. An example thereof is a method of reducing press pressure, but the process contains a problem of reduced surface smoothness and thus reduced printability. Other examples include methods of using a crosslinked pulp, blending a synthetic fiber, adding an inorganic or other filler between pulp fibers, and providing voids between them, which often result in incapability of recycling the pulp and deterioration in the smoothness of the resulting paper. A paper-bulking agent is disclosed in JP-B 2971447, but carries a problem of insufficient paper strength.
Also known is additives used during papermaking that allow improvement in the brightness, opacity, and bulking property of paper and is more effective in improving paper strength than conventional paper bulking agents (JP-A Nos. 2002-115199, 2001-248100, etc.).
In the papermaking industry, there exists a need for a paper quality improver that enables production of a bulkier paper under a high-speed high-shear papermaking condition. The high-speed papermaking is not the papermaking under a static condition wherein the pulp is diluted in a great amount of water and filtered by the weight of water as described in the conventional TAPPI papermaking technology, but the papermaking under a dynamic condition wherein the pulp slurry is supplied onto a traveling wire surface and papermade under a high-shear force in a production machine; and can be carried out in a orientational paper machine or the like in laboratory.
Alternatively, the method of adding an inorganic filler such as calcium carbonate, kaolin, white carbon, or the like in a greater amount (e.g. 5 to 20% by weight) has been also practiced in the industry for improvement in opacity and brightness. However, simple addition of an inorganic filler in a greater amount leads to increase in the weight of paper. If the amount of pulp used is reduced, addition of an inorganic filler cancels out the weight reduction and cannot achieve the reduction in the weight of paper. In particular, when an inorganic filler is added to a waste paper pulp, the amount of the inorganic filler increases, making it more difficult to achieve the reduction in the weight of paper.